Method for measuring concentration of fluorine gas contained in halogen fluoride-containing gas by ultraviolet spectroscopy

ABSTRACT

A method for measuring the concentration of fluorine gas, which includes irradiating a halogen fluoride-containing gas with ultraviolet light in which the ratio (W X /W F ) of the maximum value (W X ) of ultraviolet light intensity in the wavelength region of less than 250 nm with respect to the ultraviolet light intensity (W F ) at a wavelength of 285 nm is 1/10 or less, and measuring the absorbance at a wavelength of 285 nm to obtain the concentration of fluorine gas contained in the halogen fluoride-containing gas.

TECHNICAL FIELD

The present invention relates to a method for measuring theconcentration of fluorine gas contained in a halogen fluoride-containinggas.

BACKGROUND ART

Halogen fluorides are used in for example etching gas and cleaning gasin semiconductor manufacturing processes. In recent years, theminiaturization of semiconductors is in progress, and it is requiredthat etching gas and cleaning gas for example used in semiconductormanufacturing processes be high-purity gas. To prepare high-purity gas,a method for measuring the concentration of fluorine gas, which is animpurity, contained in etching gas and cleaning gas for example withgood accuracy is needed.

As a method for measuring the concentration of fluorine gas, forexample, Patent Literature 1 discloses a method in which theconcentration of fluorine gas contained in a gas such as exhaust gasreleased from an electronic device manufacturing apparatus such as asemiconductor manufacturing apparatus is measured by using anultraviolet spectrophotometer and a Fourier transform infraredspectrophotometer. Patent Literature 2 discloses a method in which theconcentration of fluorine in exhaust gas released from a semiconductorprocess apparatus that uses sulfur hexafluoride is measured by using anultraviolet spectrophotometer and a Fourier transform infraredspectrophotometer. Patent Literature 3 discloses a method in whichhalogen gas generated from a semiconductor manufacturing process isanalyzed by ultraviolet-visible absorption spectroscopy.

CITATION LIST Patent Literature

Patent Literature 1: JP 5221881 B2

Patent Literature 2: JP 2010-203855 A

Patent Literature 3: U.S. Pat. No. 6,686,594 B2

SUMMARY OF INVENTION Technical Problem

In the case where the concentration of fluorine gas contained in ahalogen fluoride-containing gas is measured by ultraviolet spectroscopy,since the halogen fluoride absorbs mainly light of wavelengths of lessthan 250 nm, part of the halogen fluoride is photolyzed, and fluorinemolecules or fluorine radicals are generated. Hence, there has been aproblem that, when measuring the concentration of fluorine gas in ahalogen fluoride, measurement errors occur because of fluorine orfluorine radicals generated by photolysis.

Thus, an object of the present invention is to provide a high-accuracymeasurement method that, in the measurement of the concentration offluorine gas contained in a halogen fluoride-containing gas using anultraviolet spectrophotometer, reduces the amount of measurement errorscaused by for example fluorine gas generated by photolysis of thehalogen fluoride.

Solution to Problem

The present inventor and others conducted extensive studies in order tosolve the issue mentioned above, and have found out that, when measuringthe concentration of fluorine gas contained in a halogenfluoride-containing gas by irradiating ultraviolet light, high-accuracymeasurement can be made by suppressing irradiation with ultravioletlight of wavelengths of less than 250 nm and thus have completed thepresent invention. That is, the present invention includes [1] to [8]shown below.

[1] A method for measuring a concentration of fluorine gas, the methodcomprising irradiating a halogen fluoride-containing gas withultraviolet light in which a ratio (W_(X)/W_(F)) of a maximum value(W_(X)) of ultraviolet light intensity in a wavelength region of lessthan 250 nm with respect to an ultraviolet light intensity (W_(F)) at awavelength of 285 nm is 1/10 or less, and measuring an absorbance at awavelength of 285 nm to obtain a concentration of fluorine gas containedin the halogen fluoride-containing gas.

[2] The method for measuring a concentration of fluorine gas accordingto [1], in which, using a means for suppressing irradiation withultraviolet light of wavelengths of less than 250 nm, the halogenfluoride-containing gas is irradiated with ultraviolet light ofwavelengths of not less than 250 nm from a light source.

[3] The method for measuring a concentration of fluorine gas accordingto claim 1 or 2, wherein the means is to irradiate the ultraviolet lightirradiated from the light source to the halogen fluoride-containing gasvia a filter that blocks at least 50% of the ultraviolet light with awavelength of less than 250 nm and transmits at least 90% of theultraviolet light with a wavelength of 280-290 nm.

[4] The method for measuring a concentration of fluorine gas accordingto any one of [1] to [3], in which the halogen fluoride is one kind ofgas selected from the group consisting of chlorine trifluoride, brominepentafluoride, iodine heptafluoride, bromine trifluoride, and iodinepentafluoride.

[5] The method for measuring a concentration of fluorine gas accordingto any one of [1] to [4], in which the halogen fluoride is iodineheptafluoride.

[6] The method for measuring a concentration of fluorine gas accordingto any one of [1] to [4], in which the halogen fluoride is brominepentafluoride, and the maximum value (W_(X)) of ultraviolet lightintensity in the wavelength region of less than 250 nm is a maximumvalue of ultraviolet light intensity in a wavelength region of less than225 nm.

[7] The method for measuring a concentration of fluorine gas accordingto any one of [1] to [4], in which the halogen fluoride is chlorinetrifluoride, and the maximum value (W_(X)) of ultraviolet lightintensity in the wavelength region of less than 250 nm is a maximumvalue of ultraviolet light intensity in a wavelength region of less than215 nm.

[8] The method for measuring a concentration of fluorine gas accordingto any one of [1] to [7], in which an absorption spectrum measured byirradiating a reference gas with the ultraviolet light is subtractedfrom an absorption spectrum measured by irradiating the halogenfluoride-containing gas with the ultraviolet light, and theconcentration of fluorine gas is obtained from an absorbance at awavelength of 285 nm of an obtained absorption spectrum.

Advantageous Effects of Invention

According to the present invention, high-accuracy measurement can bemade for the concentration of fluorine gas contained in a halogenfluoride-containing gas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an example of an analysis apparatusused for measurement of the concentration of fluorine gas of the presentinvention.

DESCRIPTION OF EMBODIMENTS

Hereinbelow, the present invention is described in detail, withreference to FIG. 1 as necessary. The analysis apparatus used in thepresent invention is not limited to the analysis apparatus shown in FIG.1.

The present invention relates to a method for measuring theconcentration of fluorine gas, the method comprising irradiating ahalogen fluoride-containing gas with ultraviolet light in which theratio (W_(X)/W_(F)) of the maximum value (W_(X)) of ultraviolet lightintensity in the wavelength region of less than 250 nm with respect tothe ultraviolet light intensity (W_(F)) at a wavelength of 285 nm is1/10 or less, and measuring the absorbance at a wavelength of 285 nm toobtain the concentration of fluorine gas contained in the halogenfluoride-containing gas.

<Gas to be Measured, Equipment and Others Used for Measurement of theConcentration of Fluorine Gas> (Halogen Fluoride-Containing Gas)

The halogen fluoride contained in the halogen fluoride-containing gasused in an embodiment of the present invention is a fluorine compoundcontaining a halogen such as chlorine, bromine, or iodine as aconstituent element. Examples of the halogen fluoride include chlorinefluoride, chlorine trifluoride, bromine fluoride, bromine trifluoride,bromine pentafluoride, iodine fluoride, iodine trifluoride, iodinepentafluoride, and iodine heptafluoride. Among these, in terms ofetching properties and cleaning properties, chlorine trifluoride,bromine trifluoride, bromine pentafluoride, iodine pentafluoride, andiodine heptafluoride are preferable, and chlorine trifluoride, iodineheptafluoride, and bromine pentafluoride can be used for the presentinvention more favorably. In the halogen fluoride-containing gas, onekind of halogen fluoride may be contained singly, or a plurality ofkinds may be contained.

Fluorine gas, which is to be measured, and impurity gases other thanfluorine gas may be contained in the halogen fluoride-containing gas.Examples of impurity gases include helium, argon, oxygen gas (O₂),nitrogen gas (N₂), carbon dioxide, and carbon tetrafluoride. One singlekind or a plurality of kinds of impurity gas may be contained in thehalogen fluoride-containing gas, and the amount of impurity gascontained is not particularly limited.

Further, a diluent gas may be contained in the halogenfluoride-containing gas. The diluent gas is a gas inert to the halogenfluoride, the fluorine-containing gas, and the impurity gas mentionedabove. Examples of the diluent gas include helium, argon, nitrogen gas(N₂), carbon dioxide, and carbon tetrafluoride. One single kind or aplurality of kinds of diluent gas may be contained in the halogenfluoride-containing gas, and the amount of diluent gas contained is notparticularly limited.

The halogen fluoride-containing gas is introduced into a gas cell 22described later from a halogen fluoride-containing gas supply source 10via a valve 14. The supply method, form, size, for example, of thehalogen fluoride-containing gas supply source 10 are not particularlylimited as long as the halogen fluoride-containing gas supply source 10can supply a halogen fluoride-containing gas to the gas cell 22. Forexample, a halogen fluoride-containing gas may be supplied to the gascell 22 from a branch pipe branched from a halogen fluoride-containinggas supply pipe connected to an etching apparatus used in asemiconductor manufacturing process via the valve 14, or may be suppliedto the gas cell 22 from a container such as a gas cylinder in which thesame halogen fluoride-containing gas as the gas supplied to an etchingapparatus is retained.

(Reference Gas)

When irradiating the halogen fluoride-containing gas mentioned abovewith ultraviolet light by a method described later and measuring theabsorption spectrum, it is preferable to use a reference gas as a blankand measure the absorption spectrum of the reference gas in the gas cell22 described later. The reference gas is not particularly limited aslong as the reference gas does not contain a component that absorbswavelengths around 285 nm. Examples of the reference gas includenitrogen gas (N₂) and helium gas.

The reference gas is introduced into the gas cell 22 from a referencegas supply source 12 via a valve 16. The supply method, form, size, forexample, of the reference gas supply source 12 are not particularlylimited as long as a reference gas can be supplied to the gas cell 22.For example, a reference gas may be supplied to the gas cell 22 from acontainer such as a gas cylinder in which the reference gas is retained.

(Light Source 18)

A light source 18 used in an embodiment of the present invention is usedto irradiate ultraviolet light onto the halogen fluoride-containing gasand the reference gas mentioned above. The light source 18 is notparticularly limited as long as the light source 18 is one that emitsultraviolet light including a wavelength of 285 nm; for example, thelight source such as a deuterium lamp, a xenon lamp, a mercury lamp (lowpressure or high pressure), a metal halide lamp, a fluorescent lamp, ablack light (a blue lamp) may be used, and also a light beam with fewlight components of wavelengths of less than 250 nm may be used.

(Filter 20)

In an embodiment of the present invention, when using a means forsuppressing irradiation with ultraviolet light of wavelengths of lessthan 250 nm, it is preferable that the ultraviolet light irradiated fromthe light source 18 mentioned above be irradiated onto the halogenfluoride via a filter 20, in terms of being able to suppress thephotolysis of the halogen fluoride with good efficiency. The filter 20is not particularly limited as long as the filter 20 can blockultraviolet light of wavelengths of less than 250 nm sufficiently; thefilter 20 is preferably one that can block 50% or more of ultravioletlight of wavelengths of less than 250 nm, more preferably one that canblock 60% or more, and still more preferably one that can block 70% ormore are used. By irradiating ultraviolet light via the filter 20, areduction in measurement accuracy due to for example fluorine gasgenerated by photolysis of a halogen fluoride that has the maximumabsorption wavelength in the wavelength region of less than 250 nm canbe suppressed.

As the filter 20, a filter that sufficiently transmits ultraviolet lightincluding a wavelength of 285 nm, which is the wavelength of the maximumabsorption of fluorine, is preferably used in terms of being able tomake high-accuracy measurement; it is desirable for the filter 20 totransmit preferably 90% or more of ultraviolet light of wavelengths of280 to 290 nm, and more preferably 95% or more. The filter 20 is notparticularly limited as long as the filter 20 has the propertiesmentioned above; for example, a commercially available product such as ashort wavelength cut filter manufactured by Asahi Spectra Co., Ltd. maybe used.

(Gas Cell 22)

The gas cell 22 is used to allow the halogen fluoride-containing gas andthe reference gas mentioned above to be irradiated with ultravioletlight while being enclosed or distributed. In the gas cell 22, the mainbody of the gas cell is provided with a gas inlet, a gas exhaust portconnected to an outlet 26, an entrance window, an exit window, forexample. The material of the main body of the gas cell 22 other than theentrance window or the exit window is not particularly limited as longas it is a material that exhibits corrosion resistance to the componentscontained in the halogen fluoride-containing gas and fluorine gas; forexample, stainless steel, nickel, Inconel, or Monel may be used.

The material of the entrance window and the exit window is notparticularly limited as long as it is a material that does not absorblight of wavelengths around 285 nm and that exhibits corrosionresistance to the halogen fluoride-containing gas and thefluorine-containing gas; for example, calcium fluoride or bariumfluoride may be used.

(Spectroscope 24)

A spectroscope 24 measures the absorption spectrum of the wavelengths ofultraviolet light emitted from the exit window of the gas cell 22mentioned above. The spectroscope 24 is not particularly limited as longas the spectroscope 24 can measure the absorption spectrum ofultraviolet light; for example, an ultraviolet spectrophotometercommonly used in the field of the present invention may be used.

(Others)

In order to take light sent from the light source 18 into the gas cell22 with good efficiency, a lens for example may be provided between theemission end of the light source 18 and the entrance window of the gascell 22.

<Method for Measuring the Concentration of Fluorine Gas>

Hereinbelow, a method for measuring the concentration of fluorine gas ofthe present invention using the gas to be measured, the equipment andothers described above is described.

(1) Measurement of the Reference Gas

The absorption spectrum of the reference gas is preferably measured inadvance in order to measure the concentration of fluorine gas containedin the halogen fluoride-containing gas of the present invention. Tomeasure the absorption spectrum of the reference gas, the valve 14 isclosed to prevent halogen fluoride-containing gas from being suppliedfrom the halogen fluoride-containing gas supply source 10, and the valve16 is opened to introduce the reference gas into the gas cell 22 fromthe reference gas supply source 12. The introduction of the referencegas may be performed such that the reference gas is distributed throughthe gas cell 22 while the outlet 26 is kept open, or may be performedsuch that the outlet 26 is closed and the reference gas is enclosed inthe gas cell 22.

Ultraviolet light emitted from the light source 18 is irradiated ontothe reference gas in the gas cell 22 from the entrance window of the gascell 22, preferably via the filter 20. At this time, the ultravioletlight of the light source 18 may be irradiated onto the entrance windowof the gas cell 22 via an optical fiber. Ozone may be generated fromoxygen in the air because of ultraviolet light of short wavelengths, andmay affect the measurement; thus, it is preferable that a purge gas suchas nitrogen be supplied to and around the entrance window or that anairtight structure be employed so that oxygen and air for example do notget in from the entrance window or the exit window.

Moreover, it is necessary that the ratio (W_(X)/W_(F)) (hereinafter,also referred to as “the ultraviolet light intensity ratio”) of themaximum value (W_(X)) of ultraviolet light intensity in the wavelengthregion of less than 250 nm with respect to the ultraviolet lightintensity (W_(F)) at a wavelength of 285 nm of ultraviolet lightirradiated from the light source 18 onto the reference gas and thehalogen fluoride-containing gas be 1/10 or less. Thereby, a situationwhere the halogen fluoride is photolyzed by irradiation with ultravioletlight of wavelengths of less than 250 nm and the resulting fluorine gasreduces measurement accuracy can be suppressed. The ultraviolet lightintensity ratio (W_(X)/W_(F)) mentioned above is more preferably 1/15 orless. The maximum value of ultraviolet light intensity in the wavelengthregion of less than 250 nm is, in other words, the intensity ofultraviolet light at a wavelength of the highest intensity out ofultraviolet light of wavelengths of less than 250 nm. The W_(E) andW_(X) mentioned above can be measured by the spectroscope 24 mentionedabove. The ultraviolet light intensity ratio mentioned above can beadjusted with the light source 18 and the filter 20.

Within the range mentioned above, the ultraviolet light intensity ratiomentioned above may be set in accordance with the wavelength ofabsorption of the halogen fluoride, as appropriate. For example, thewavelength of the maximum absorption of bromine pentafluoride is 217 nm;the maximum value of ultraviolet light intensity of the wavelengthregion shorter than a wavelength of the maximum absorption wavelengthmentioned above plus 7 to 9 nm, for example the maximum value ofultraviolet light intensity of the wavelength region of less than 225nm, may be taken as W_(X), and may be used for the calculation of theultraviolet light intensity ratio.

Similarly, the wavelength of the maximum absorption of chlorinetrifluoride is 207 nm; thus, for example, the maximum value ofultraviolet light intensity of the wavelength region of less than 215 nmmay be taken as W_(X), and may be used for the calculation of theultraviolet light intensity ratio. Further, the wavelength of themaximum absorption of iodine heptafluoride is 241 nm; thus, for example,the maximum value of ultraviolet light intensity of the wavelengthregion of less than 250 nm may be taken as W_(X), and may be used forthe calculation of the ultraviolet light intensity ratio.

Ultraviolet light emitted from the exit window of the gas cell 22 ismeasured with a spectroscope. The ultraviolet light emitted from theexit window may be introduced into the spectroscope via an opticalfiber. When using a spectroscope, the absorption spectrum of thereference gas is measured by, for example, performing operation inconformity with a manual that comes with the product. When, for example,high-purity nitrogen gas is used as the reference gas and the referencegas is irradiated with ultraviolet light while using a filter so as toachieve the range of the ultraviolet light intensity ratio mentionedabove, the absorption spectrum can be used as a blank because there isno absorption of nitrogen or the halogen fluoride in the wavelengthregion of less than 250 nm.

(2) Measurement of the Absorbance at a Wavelength of 285 nm of theHalogen Fluoride-Containing Gas

The valve 16 is closed, the valve 14 is opened to introduce the halogenfluoride-containing gas into the gas cell 22 from the halogenfluoride-containing gas supply source 10, and the absorption spectrum ofthe halogen fluoride-containing gas is measured similarly to that of thereference gas mentioned above. At this time, a halogenfluoride-containing gas diluted with a diluent gas such as helium,argon, nitrogen, carbon dioxide, or carbon tetrafluoride may beintroduced into the gas cell 22. The absorbance at a wavelength of 285nm, which is the wavelength of the maximum absorption of fluorine, ismeasured from the absorption spectrum, and the concentration of fluorinegas is obtained by absorption spectrophotometry.

In terms of being able to make high-accuracy measurement, it ispreferable that the absorption spectrum of the reference gas mentionedabove be subtracted from the absorption spectrum of the halogenfluoride-containing gas, the absorbance at a wavelength of 285 nm bemeasured from the obtained absorption spectrum, and the concentration offluorine gas be obtained by absorption spectrophotometry.

(3) Measurement Conditions

The temperature in the gas cell 22 when performing the measurementmentioned above is not particularly limited as long as it is not lessthan the temperatures at which the halogen fluoride-containing gas andthe reference gas liquefy and solidify, but is preferably 20 to 150° C.,and more preferably 50 to 120° C. If the temperature is higher than thisrange, the reaction between the halogen fluoride and each of the gascell, and the entrance window and the exit window may progress, or thedecomposition of the halogen fluoride may progress; thus, this is notpreferable.

The pressure in the gas cell 22 when enclosing the halogenfluoride-containing gas and the reference gas mentioned above in the gascell 22 and performing measurement is not particularly limited, but ispreferably 0.01 to 0.2 MPaA, and more preferably 0.05 to 0.15 MPaA. Ifthe pressure is lower than this range, the gas concentration mentionedabove may be lowered, and sensitivity may be reduced; if the pressure ishigher than this range, the apparatus may be damaged.

EXAMPLES

Hereinbelow, the present invention is described still more specificallyon the basis of Examples; however, the present invention is not limitedto these Examples.

Example 1

A bromine pentafluoride gas was used as the halogen fluoride-containinggas; using the analysis apparatus shown in FIG. 1, the concentration offluorine gas contained in the bromine pentafluoride gas was measured inconformity with the measurement method of the present invention. As thegas cell 22, a gas cell in which the main body is made of SUS 316 andthe entrance window and the exit window are formed of calcium fluoridewas used.

First, nitrogen gas was used as the reference gas; nitrogen gas wasintroduced into the gas cell 22 from a high-purity nitrogen gascylinder, which is the reference gas supply source 12; the maximum valueof ultraviolet light intensity of the wavelength region of less than 225nm was taken as W_(X), and ultraviolet light of W_(X)/W_(F)=1/20 wasirradiated onto the nitrogen gas in the gas cell 22 from a deuteriumlamp (product name: L10290, manufactured by Hamamatsu Photonics K.K.) asthe light source 18 via a short wavelength cut filter (product name:LU0250, manufactured by Asahi Spectra Co., Ltd.) as the filter 20. Theabsorption spectrum of ultraviolet light emitted from the gas cell 22was measured with a multichannel spectroscope (product name: FLAME-S,manufactured by Ocean Optics, Inc.) as the spectroscope 24. Thetemperature in the gas cell was 50° C., and the pressure was 0.1 MPaA.

Next, the nitrogen gas in the gas cell 22 was discharged from the outlet26; then, bromine pentafluoride gas was introduced into the gas cell 22from the halogen fluoride-containing gas supply source 10; and under theconditions of the same temperature and the same pressure as those at thetime of the reference gas measurement, ultraviolet light ofW_(X)/W_(F)=1/20 was irradiated onto the bromine pentafluoride gas inthe gas cell 22 from the light source via the filter mentioned above.Here, the absorption spectrum of ultraviolet light emitted from the gascell 22 was measured with the spectroscope mentioned above. Theabsorption spectrum of the nitrogen gas was subtracted from the obtainedabsorption spectrum of the bromine pentafluoride gas, and theconcentration of fluorine gas contained in the bromine pentafluoride gaswas found. As a result, it was found that the concentration of fluorinewas 2 volume ppm.

Example 2

The concentration of fluorine gas in iodine heptafluoride gas was foundsimilarly to Example 1 except that iodine heptafluoride gas was used inplace of the bromine pentafluoride gas as the halogenfluoride-containing gas, the maximum value of ultraviolet lightintensity of the wavelength region of less than 250 nm was taken asW_(X), and ultraviolet light of W_(X)/W_(F)=1/18 was irradiated. As aresult, it was found that the concentration of fluorine was 3 volumeppm.

Example 3

The concentration of fluorine gas in chlorine trifluoride gas was foundsimilarly to Example 1 except that chlorine trifluoride gas was used inplace of the bromine pentafluoride gas as the halogenfluoride-containing gas and the maximum value of ultraviolet lightintensity of the wavelength region of less than 215 nm was taken asW_(X). As a result, it was found that the concentration of fluorine was5 volume ppm.

Comparative Example 1

The concentration of fluorine gas contained in bromine pentafluoride gaswas found similarly to Example 1 except that the filter was not used andultraviolet light of W_(X)/W_(F)=1/5 was irradiated. As a result, it wasfound that the concentration of fluorine was 20 volume ppm and thedecomposition reaction of bromine pentafluoride had progressed.

Comparative Example 2

The concentration of fluorine gas contained in iodine heptafluoride gaswas found similarly to Example 2 except that the filter was not used andultraviolet light of W_(X)/W_(F)=1/5 was irradiated. As a result, it wasfound that the concentration of fluorine was 24 volume ppm and thedecomposition reaction of iodine heptafluoride had progressed.

Comparative Example 3

The concentration of fluorine gas contained in chlorine trifluoride gaswas found similarly to Example 3 except that the filter was not used andultraviolet light of W_(X)/W_(F)=1/5 was irradiated. As a result, it wasfound that the concentration of fluorine was 18 volume ppm and thedecomposition reaction of chlorine trifluoride had progressed.

The conditions and results of Examples 1 to 3 and Comparative Examples 1to 3 described above are shown in Table 1.

TABLE 1 Ratio of ultraviolet light Fluorine Halogen fluoride - intensityconcentration containing gas (W_(X)/W_(F)) (vppm) Example 1 brominepentafluoride 1/20 2 Example 2 iodine heptafluoride 1/18 3 Example 3chlorine trifluoride 1/20 5 Comparative bromine pentafluoride 1/5  20example 1 Comparative iodine heptafluoride 1/5  24 example 2 Comparativechlorine trifluoride 1/5  18 example 3

REFERENCE SIGNS LIST

-   10 halogen fluoride-containing gas supply source-   12 reference gas supply source-   14 valve-   16 valve-   18 light source-   20 filter-   22 gas cell-   24 spectroscope-   26 outlet

1. A method for measuring a concentration of fluorine gas, the methodcomprising: irradiating a halogen fluoride-containing gas withultraviolet light in which a ratio (W_(X)/W_(F)) of a maximum value(W_(X)) of ultraviolet light intensity in a wavelength region of lessthan 250 nm with respect to an ultraviolet light intensity (W_(F)) at awavelength of 285 nm is 1/10 or less; and measuring an absorbance at awavelength of 285 nm to obtain a concentration of fluorine gas containedin the halogen fluoride-containing gas.
 2. The method for measuring aconcentration of fluorine gas according to claim 1, wherein using ameans for suppressing irradiation with ultraviolet light of wavelengthsof less than 250 nm, the halogen fluoride-containing gas is irradiatedwith ultraviolet light of wavelengths of not less than 250 nm from alight source.
 3. The method for measuring a concentration of fluorinegas according to claim 1, wherein the means is to irradiate theultraviolet light irradiated from the light source to the halogenfluoride-containing gas via a filter that blocks at least 50% of theultraviolet light with a wavelength of less than 250 nm and transmits atleast 90% of the ultraviolet light with a wavelength of 280-290 nm. 4.The method for measuring a concentration of fluorine gas according toclaim 1, wherein the halogen fluoride is one kind of gas selected fromthe group consisting of chlorine trifluoride, bromine pentafluoride,iodine heptafluoride, bromine trifluoride, and iodine pentafluoride. 5.The method for measuring a concentration of fluorine gas according toclaim 1, wherein the halogen fluoride is iodine heptafluoride.
 6. Themethod for measuring a concentration of fluorine gas according to claim1, wherein the halogen fluoride is bromine pentafluoride, and themaximum value (W_(X)) of ultraviolet light intensity in the wavelengthregion of less than 250 nm is a maximum value of ultraviolet lightintensity in a wavelength region of less than 225 nm.
 7. The method formeasuring a concentration of fluorine gas according to claim 1, whereinthe halogen fluoride is chlorine trifluoride, and the maximum value(W_(X)) of ultraviolet light intensity in the wavelength region of lessthan 250 nm is a maximum value of ultraviolet light intensity in awavelength region of less than 215 nm.
 8. The method for measuring aconcentration of fluorine gas according to claim 1, wherein anabsorption spectrum measured by irradiating a reference gas with theultraviolet light is subtracted from an absorption spectrum measured byirradiating the halogen fluoride-containing gas with the ultravioletlight, and the concentration of fluorine gas is obtained from anabsorbance at a wavelength of 285 nm of an obtained absorption spectrum.